Abstract/Summary The overall objective of this proposal is to investigate the regulation of gonadotropin-releasing hormone (GnRH) neurons in the non-human primate. GnRH is released from the hypothalamus and controls reproductive function through pituitary gonadotropins. The proposed studies are designed to uncover the role of neuroestradiol in the preovulatory GnRH surge. It has been shown for many years that circulating E2 released from the ovaries facilitates learning and memory, protects neurons from neuronal cell death, and regulates reproductive function. However, more recently, a new concept regarding the role of E2, synthesized and released locally in the brain, as a neuromodulator of neural functions, has emerged. In fact, our preliminary data indicate that E2 synthesized and released in the stalk- median eminence (S-ME) of the hypothalamus appears to be necessary for the full gonadotropin surge. That is, estradiol benzoate (EB)-induced LH surge in ovariectomized (OVX) female monkeys was greatly attenuated in the presence of the aromatase blocker letrozole. Aromatase is the enzyme necessary for E2 synthesis from androgens and letrozole is a commonly used competitive blocker for aromatase synthesis. In the proposed project, we will test the central hypothesis that neuroestradiol, synthesized and released in the hypothalamus plays a critical role in regulation of preovulatory GnRH release. Three Specific Aims are proposed. Aim 1 will test the hypothesis that neuroestradiol is an integral part of the estrogen's positive feedback influence on GnRH release. In Aim 1, we will assess the timing of neuroestradiol increases during the EB-induced LH surge using letrozole injection as a tool and measuring the changes in LH release. Aim 2 will test the hypothesis that neuroestradiol increases during the EB-induced GnRH surge are an underling mechanism of the sustained elevation of GnRH release. In Aim 2 we will measure release of estradiol and GnRH as well as circulating LH and E2 in EB treated OVX monkeys using a microdialysis method and serial blood sampling followed by analysis with liquid chromatography-mass spectrometry (LC/MS/MS) and RIA. Aim 3 will test the hypothesis that aromatase activity in the hypothalamus increases during the preovulatory GnRH surge in vivo. To visualize aromatase we will use positron emission tomography (PET) scan with 11C-labeled cetrozole as a marker. The proposed study has great potential to demonstrate that E2 synthesized in the hypothalamus increases during the preovulatory gonadotropin surge in vivo. In turn, this finding will modify the presently accepted dogma that E2 of ovarian origin solely controls the hypothalamo-pituitary-gonadal axis.